MALACOSTEACA 969 



whilst the tubular structure of the thick inner layer may be readily 

 demonstrated by means of sections parallel and perpendicular to its 

 surface. This structure, which resembles that of dentine, save that 

 the tubuli do not branch, but remain of the same size through their 

 whole course, may be particularly well seen in the black extremity 

 of the claw, which (apparently from some peculiarity in the mole- 

 cular arrangement of its mineral particles) is much denser than the 

 rest of the shell, the former having almost tin- semi-transparence 

 of ivory, whilst the latter has a chalky opacity. In a transverse 

 section of the claw the tubuli may be seen to radiate from the central 

 cavity towards the surface, so as very strongly to resemble their 

 arrangement in a tooth ; and the resemblance is still further increased 

 by the presence, at tolerably regular intervals, of minute sinuosities 

 corresponding with the laminations of the shell, which seem, like 

 the ' secondary curvatures ' of the dentinal tubuli, to indicate suc- 

 cessive stages in the calcification of the animal basis. In thin 

 sections of the areolated layer it may be seen that the apparent 

 walls of the areola? are merely translucent spaces from which the 

 tubuli are absent, their orifices being abundant in the intervening 

 spaces. 1 The tubular layer rises up through the pigmentary layer 

 of the crab's shell in little papillary elevations, which seem to be 

 concretionary nodules; and it is from the deficiency of the pig- 

 mentary layer at these parts that the coloured portion of the shell 

 derives its minutely speckled appearance. Many departures from 

 this type are presented by the different species of decapods ; thus 

 in the prawns there are large stellate pigment-spots resembling 

 those of frogs, the colours of which are often in remarkable con- 

 formity with those of the bottom of the rock-pools frequented by 

 these creatures ; whilst in the shrimps there is seldom any distinct 

 trace of the areolated layer, and the calcareous portion of the skele- 

 ton is disposed in the form of concentric rings, which seem to be the 

 result of the concretionary aggregation of the calcifying deposit. 2 



It is a very curious circumstance that a strongly marked dif- 

 ference exists between crustaceans that are otherwise very closely 

 allied in regard to the degree of change to which their young are 

 subject in their progress towards the adult condition. For, whilst 

 the common crab, lobster, spiny lobster, prawn, and shrimp 

 undergo a regular metamorphosis, the young of the crayfish and 

 some land-crabs come forth from the egg in a form which corre- 

 sponds in all essential particulars with that of their parents. 

 Generally speaking, a strong resemblance exists among the young 

 of all the species of decapods which undergo a metamorphosis, whether 

 they are afterwards to belong to the inacrurons (long-tailed) or to 

 the lii-iicjii/iii-nii.s (short-tailed) division of the group; and the forms 



1 The Author is now quite satisfied of the correctness of the interpretation put by 

 Professor Huxley (see his article, ' Teguuientary Organs,' in the Cyclop. Anui. anil 

 Phtjs. vol. v. p. 487), and by Professor W. C. Williamson ( ' On some Histological 

 Features in the Shells of Crustacea ' in Quart. Journ. Micrnsc. St.-i. vol. viii. 1860, 

 p. -'S) upon the appearances which he formerly described {Report of British Asso- 

 ciation for 1847, p. 128) as indicating a cellular structure in this layer. 



2 Consult Braun, ' Ueber die histologischen Vorgange bei der Hautuug von 

 Astacus fluviatilis,' Arbeit. ZiioL I/t*t. Wiirzbin-g, ii. p. 121. 



